In an optical spectrum analyzer or an optical power meter which measures the intensity of light, an I/V (current/voltage) conversion type photoelectric conversion circuit which outputs a voltage signal proportional to the intensity of incident light is used.
As shown in FIG. 6, the I/V conversion type photoelectric conversion circuit includes a photodiode 11, which has a cathode connected to a positive voltage Va(+) of a power supply and which outputs a current Ip proportional to the intensity of incident light from its anode side, and an inverting amplifier 12 using an operational amplifier 12a, which has a non-inverting input terminal grounded, and a feedback resistor 12b. By supplying the output current Ip of the photodiode 11 to the input of the inverting amplifier 12, a voltage Vout equivalent to “−Ip×R” is output at the output terminal.
In the inverting amplifier using the operational amplifier, the inverting input terminal and the non-inverting input terminal are virtually short-circuited. When the non-inverting input terminal is grounded and the electric potential is fixed to 0 V, the electric potential of the inverting input terminal also becomes 0 V and the input resistance is too large. Accordingly, the current Ip flowing from the photodiode 11 flows to the output terminal through the feedback resistor 12b of the operational amplifier 12a. As a result, the voltage Vout equivalent to “−Ip×R” is output.
In such an I/V conversion type photoelectric conversion circuit, the dynamic range of the photodiode 11 generally has a size of 80 dB or more in terms of input light power (the same hereinbelow). However, since the effective dynamic range at the amplifier side is about 30 to 40 dB, it is necessary to perform range switching at the amplifier side when performing photoelectric conversion corresponding to the entire dynamic range of the photodiode 11.
For example, as shown in FIG. 7, two feedback resistors 12c and 12d with different resistance values Ra and Rb (Ra<Rb) are set so as to be selectable by a switch 12e. In this case, in a range (non-saturated region) where the output current Ip of the photodiode 11 is lower than a certain threshold value Ith (for example, Ith=Vb(−)/Rb) due to the low intensity of incident light, the output voltage of Vout=−Ip×Rb is obtained using the feedback resistor 12d with the larger resistance value.
Moreover, in a range where the output current Ip of the photodiode 11 exceeds the threshold value Ith due to the high intensity of incident light (however, not exceeding Vb(−)/Ra), the output voltage of Vout=−Ip×Ra is obtained using the feedback resistor 12c with the smaller resistance value. Here, it is assumed that Vb(+) and Vb(−) are supply voltages (the same positive and negative voltages) of the inverting amplifier 12 and are equal to the saturation power voltage.
For example, assuming that Ra=100Ω, Rb=1 MΩ, and supply voltage of an amplifier=saturation voltage=Vb(−)=−10 V, a non-saturated output voltage up to −10 V can be obtained for the current Ip equal to or lower than |Vb(−)|/Rb=10 V/1 MΩ=10 μA when the feedback resistor 12d with the resistance value Rb is connected.
In addition, when the feedback resistor 12c with the resistance value Ra is connected, a non-saturated output voltage up to −10 V can be obtained for the current Ip equal to or lower than |Vb(−)|/Ra=10 V/100Ω=100 mA.
When the effective dynamic range of the amplifier is set to 40 dB (10000 times) in consideration of the level of residual noise and the like, the effective output range becomes −10 V to −1 mV. In this case, since the output voltage −1 mV when the feedback resistor 12d is connected is equivalent to 1 mV/1 MΩ=1 nA, the output voltage of −1 mV to −10 V is obtained for a range of 1 nA to 10 μA of the output current Ip of the photodiode.
In addition, since the output voltage −1 mV when the feedback resistor 12c is connected is equivalent to 1 mV/100Ω=10 μA, the output voltage of −1 mV to −10 V is obtained for a range of 10 μA to 100 mA of the output current Ip of the photodiode.
Accordingly, when the intensity of incident light is in a range of 1 nA to 10 μA in terms of an output current of the photodiode 11, the switch 12e is connected to the feedback resistor 12d (high gain side) and the output voltage of the operational amplifier 12a at that time is selected as an effective output. When the intensity of incident light is in a range of 10 μA to 100 mA in terms of an output current of the photodiode 11, the switch 12e is connected to the feedback resistor 12c (low gain side) and the output voltage of the operational amplifier 12a at that time is selected as an effective output.
Here, the output when the feedback resistor 12c is connected is reduced to Ra/Rb (=1/10000) of the output when the feedback resistor 12d is connected. Therefore, in order to obtain an output corresponding to the actual intensity of incident light, it is necessary to perform conversion as Rb/Ra times using a subsequent operation circuit (absolute value processing for removing a negative sign is also performed when necessary).
In addition, a technique of expanding the dynamic range at the amplifier side by switching (ranging) between feedback resistors of an I/V conversion circuit as described above is disclosed in JP-A-2007-300340, for example.